Known designs of a hydrostatic axial piston machine are the band axis motor and the swash plate motor. Such axial piston machines can be used as a hydrostatic pump or as a hydrostatic motor with an essentially identical structure. In the case of a band axis motor, a drive flange is rigidly connected to the allocated drive shaft. The piston rods of multiple displacement pistons are mounted in a jointed arrangement so that they are evenly distributed peripherally on the drive flange. The displacement pistons are each disposed in one of several axial cylinder bores correspondingly disposed such that they are peripherally distributed in a cylinder block. The cylinder block is drivingly connected to the drive shaft, for example by means of a central cardan shaft, and is rotatably mounted about its axis of rotation in a yoke, by means of the pivoting of which about a pivot axis extending perpendicular to the central axis of the drive shaft, the displacement volume of the band axis motor can be modified or adjusted. The skew angle range of a band axis motor is 45° (+/−5°) for a one-sided adjustment and is +/−45° (+/−5°) for a double-sided adjustment, i.e. with direction reversal.
In the case of a swash plate motor, a cylinder block, with multiple axial cylinder bores disposed such that they are peripherally distributed thereon, is rigidly connected to the allocated drive shaft. A displacement piston is disposed in each of the cylinder bores, the piston rod of each displacement piston being in sliding contact with an axially adjacent swash plate by means of a sliding shoe. The swash plate is rigidly or integrally connected to a yoke, by means of the pivoting of which about a pivot axis extending perpendicular to the central axis of the drive shaft, the displacement volume of the swash plate motor can be modified or adjusted. The skew angle range of a swash plate motor is 20° (+/−5°) for a one-sided adjustment and is +/−20° (+/−5°) for a double-sided adjustment, i.e. with direction reversal.
For example, a hydrostatic module is disclosed in DE 10 2008 002 140 A1 with reference to DE 10 2006 025 347 B3, which comprises two band axis motors that are hydraulically coupled with one another, the cylinder blocks of which band axis motors are disposed in a pivotable yoke so that they are offset to one another with respect to a common pivot axis. One of the band axis motors operates as a hydrostatic pump and can be driven by a drive motor by means of the allocated drive shaft. The other band axis motor operates as a hydrostatic motor and can deliver torque to a consumer load by means of an allocated drive shaft. Because of the offset arrangement of the cylinder blocks, when the yoke pivots from a neutral resting position, the displacement volume of the hydrostatic pump is increased, and the displacement volume of the hydrostatic motor is reduced to the same extent. The hydrostatic module thus forms a steplessly adjustable hydrostatic transmission, which, for example, could be used in wheel-mounted front-end loaders as the only gear box and in tractors in conjunction with a manual transmission disposed parallel to that hydrostatic module as a load-sharing transmission.
The adjusting device of the hydrostatic module disclosed in DE 10 2008 002 140 A1, in which the yoke can be pivoted about its pivot axis and therefore the gear ratio of the hydrostatic transmission can be adjusted, comprises a hydraulic adjusting cylinder with an adjusting piston such that the hydraulic adjusting cylinder is axially disposed centered between the two band axis motors in a base plate in which the bearings of the yoke are integrated radially spaced apart from one another, and tangential with respect to the pivot axis of the yoke. The adjusting piston is in operative connection with the yoke by means of a pivot lever that is radially aligned with respect to the pivot axis of the yoke, rigidly connected to the yoke, and slidably and rotatably mounted by means of a ball end in a radial bore of the adjusting piston. Thus an axial displacement of the adjusting piston causes the yoke to pivot about its pivot axis, and therefore causes an adjustment of the displacement volume of the two band axis motors. For the axial displacement of the adjusting piston, this piston can be acted on by means of a valve arrangement, not described in greater detail here, at two opposing piston surfaces with an adjusting pressure difference of a hydraulic pressure medium.
In contrast to other possible valve arrangements for controlling the adjusting cylinder, the present invention starts with a control arrangement having a relatively simple design, which comprises a position regulating valve in the form of an electromagnetically controllable hydraulic proportional valve having a control piston, by means of the axial position of which the adjusting pressure difference acting on the adjusting piston of the adjusting cylinder can be set, and a non-positive locking mechanical return device, by means of which the pivoting position of the yoke can be transferred to the control piston of the position regulating valve.
A control arrangement of this kind, with a pressure-controlled position regulating valve, is disclosed in U.S. Pat. No. 5,205,201 A, the position regulating valve being disposed axially parallel to the adjusting cylinder and being operatively connected to the adjusting piston of the adjusting cylinder by means of a non-positive locking mechanical return device. The return device comprises a connecting rod, a casing and a return spring in the form of a helical spring. The casing is disposed inside the valve housing of the position regulating valve in an expanded pressure space coaxially over the return spring as well as an extended portion of the control piston, and is connected to the adjusting piston of the adjusting cylinder on one side by means of a connecting rod, and to the control piston of the position regulating valve on the other side by means of the return spring.
DE 196 53 165 C2 discloses such a control arrangement having an electromagnetically controllable position regulating valve, which is disposed coaxially to and axially adjacent to the adjusting cylinder, and which is operatively connected to the adjusting piston of the adjusting cylinder by means of a non-positive locking mechanical return device. The return device comprises a return spring in the form of a helical spring, which is disposed inside the adjusting cylinder between the adjusting piston and the control piston of the adjacent position regulating valve.
Lastly, DE 101 38 554 C1 discloses such a control arrangement having two pressure-controlled or electromagnetically controllable position regulating valves, which are eccentrically disposed with a radial alignment adjacent to the adjusting cylinder, and each of which being operatively connected to the adjusting piston of the adjusting cylinder by means of a non-positive locking mechanical return device. The return device comprises a sensing piston adjacent to an eccentric conical portion of the adjusting piston and a return spring disposed between the respective sensing piston and the control piston of the allocated position regulating valve.
In the case of the known control arrangements, it is not the pivoting position of the respective hydrostatic module, but rather the control position of the adjusting piston of the adjusting cylinder that is operatively connected to the yoke that can be transferred to the control piston of the allocated position regulating valve by means of the return device. In this way, an unavoidable, existing play in the control connection between the adjusting piston of the adjusting cylinder and the yoke is not registered, which leads to a certain degree of imprecision in the position control of the yoke. Likewise in this way, the progressive gear ratio of the control connection between the adjusting piston of the adjusting cylinder and the yoke is not registered, which leads to an increased rate of change in the pivot angle of the yoke when there is increased deflection of the adjusting piston, which inevitably leads to an increased imprecision in the position control of the yoke with an increased pivoting angle of the yoke.